The present invention relates to test and measurement of cable television (CATV) systems, and more particularly for a cable television test and measurement system that performs measurements at the cable headend, transmits the measurements as data over the cable television system, and compares the cable headend measurements with measurements performed in the field to determine characteristics of the cable television system.
Broadband CATV systems typically include active broadband amplifiers and passive connectors, splitters and taps, all interconnected by a significant amount of cable, either coaxial or fiber-optic. The majority of these components are in outside locations, exposed to temperature and weather extremes. Proper performance of such a CATV system is critical for customer satisfaction and continued regulatory compliance, and is affected by these extremes. Therefore the frequency response of the system, including all passive and active components, is important.
Existing CATV sweep systems for determining the characteristics of a CATV system use one of the following three common techniques:
1--measurement of the available television signals in the field. This technique is limited by level errors at the CATV headend. PA1 2--measurement in the field of a test signal inserted at the headend in a guardband between channels. This technique fails because common television receivers have inadequate selectivity to reject the test signal. PA1 3--measurement in the field of a low-level in-band test signal. This technique causes interference because sound carrier demodulation depends on video carrier presence. PA1 *interference with cable signals; PA1 *insufficient "sweep-to-noise" after several amplifiers; PA1 *delayed response leading to "rubber screwdriver" effect; and PA1 *too few data points across the system bandwidth.
The approach for determining frequency response by injecting a test signal at the headend of the CATV system has the test signal sweeping across the entire system bandwidth. Simultaneously the signal amplitude is measured at various points along the system to determine system gain and flatness. Historically high level, low level, intermediate level, and even "sweepless" sweep approaches have been tried. The difficulty with these approaches lie not in actually performing the measurements, but rather in performing them while the system is operating without degrading the video signals being transmitted on the system. All these approaches suffer from various shortcomings including:
One solution, incorporated in the 2721/2722 Non-interfering Sweep System manufactured by Tektronix, Inc. of Beaverton, Oreg., United States of America, is to transmit short test pulses, approximately 8 microseconds in duration, during the vertical blanking interval of the video signals being carded by the CATV system. Since there is no video information transmitted during the vertical blanking interval, the picture quality theoretically is unaffected. The test pulse amplitude is set close to that of the system carriers, such as 6 dB down from the horizontal sync tip amplitude, so the pulses do not get lost in system noise. The measured amplitude of these pulses are compiled to show the frequency response of the CATV system.
Once the measuring signal, which is generally an RF pulse, is inserted into the vertical interval, the question becomes one of determining an appropriate amplitude so the sweeper is truly non-interfering. If the amplitude of the RF pulse is set too large, it causes interference in the sound channel of the customer's set that sounds like ignition noise. On the other hand if the amplitude is set too small, the measured results have uncertainties caused by interference from the video signal in the channel being measured.
The mechanism of sound channel interference is fairly straight forward. Consumer receivers commonly recover sound information using an intercarrier process that is critically dependent upon the phase relationship between the video and audio carriers. When the customer's set receives a video signal together with the RF pulse, it treats the RF pulse as if it were part of the video signal. Since the added RF pulse is not phase coherent with the video carrier, it introduces phase errors into the intercarrier sound demodulation process. If the amplitude of the RF pulse with respect to the video carrier is too large, the resulting phase errors cause some consumer receivers to exhibit interference effects. These effects may be noticed as a buzz caused by the RF test pulse. In the case of very short pulses, such as the RF pulse, the buzz is reduced to a "pop."
A further potential problem with sending a signal that exceeds sync tip amplitude is the compression and distortion it causes in the customer's receiver. There are no guarantees of how much extra amplitude over the sync tip level the receiver's IF output stage can handle. The receiver's AGC loop may incorrectly set the video signal in the IF stage. Thus the RF pulse amplitude should be low enough so that receiver system standards are not violated. An attempt to address this problem is disclosed in U.S. Pat. No. 5,233,418 issued Aug. 3, 1993 to Linley F. Gumm et al entitled "CATV Sweep System Using a Gated Receiver." Another attempted solution to the problem of CATV system testing on a non-interfering basis is disclosed in U.S. Pat. No. 4,408,227 issued to Bradley on Oct. 4, 1983 entitled "Method and Apparatus for Television Distribution System Sweep Testing." The Bradley system uses time division multiplexing between the test signal and the video signal as a means of eliminating interference to the video signal due to testing. The video signal provides synchronizing signals that enable the video signal to be suppressed during the vertical interval and the test signal to be inserted in its place. At the receiver the process is reversed to demultiplex out the test signal for display. However, the Bradley system causes a loud buzz on the customer's receiver due to dropping the sound carrier when the test signal is multiplexed with the video signal. This is similar to the buzz caused by intercarrier phase distortion mentioned above.
All of the above fail to address the phase shift in the video carrier caused by the addition of the RF test signal to the video signal. This phase shift introduces a similar phase shift into the sound signal when the sound carrier is demodulated. The result is still a distortion that occurs on the sound channel at the customer's television receiver. Further with proposed digital television standards there is no vertical interval into which test signals may be inserted.
What is desired is a cable television test and measurement system that determines the characteristics of the system in-service without introducing any video or sound distortions into the television receiver regardless of whether the video is analog or digital.